The ‘Science’ of Global Warming, Part 2

Context and clarity are needed to understand the UN IPCC’s latest report on climate change.

In late September, the media rushed to judgment in its reporting on the new United Nations Intergovernmental Panel on Climate Change report. Yesterday and today, The American examines the fine print, with essays by environmental scientist Kenneth Green and economist Benjamin Zycher.

The United Nations Intergovernmental Panel on Climate Change (IPCC) has released its Fifth Assessment Report (AR5) Summary for Policymakers (SPM), and in terms of this official view of climate science, the beginning of wisdom is straightforward: it is a political document first and a (partial) summary of the scientific literature only secondarily. That is the only conclusion consistent with the following “Disclaimer” published on the IPCC website:

The Summary for Policymakers will be released on Friday, 27 September 2013. The accepted Final Draft of the full Working Group I report, comprising the Technical Summary, 14 Chapters and three Annexes, will be released online in unedited form on Monday 30 September. Following copy-editing, layout, final checks for errors, and adjustments for changes for consistency with the Summary for Policymakers, it will be published online in January 2014. (Emphasis added)

Got that? The technical report will be adjusted to make it consistent with the SPM, rather than the reverse. Accordingly, in a draft of the SPM released (or leaked) last June, IPCC noted that the climate “models do not generally reproduce the observed reduction in surface warming trend over the last 10-15 years.”1 That statement is nowhere to be found in the new version of the SPM, and only the weak substitute quoted immediately below has replaced it. In short: IPCC now essentially is shunting aside the absence of a temperature trend since roughly 2000, one that is inconsistent with the predictions of the models and with the assertions about the effects of increasing greenhouse gas (GHG) concentrations made in the previous four assessment reports. Instead, we have the following obfuscation:2

The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phases of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effect of aerosols). (Italics in the original)

Wow. IPCC is arguing — simultaneously — that the recent absence of warming is due “in roughly equal measure” to reduced radiative forcing and to internal variability, and that there is “low confidence” in quantifying the role of changes in radiative forcing. And — again simultaneously — that “internal decadal variability causes to a substantial degree the difference between observations and the simulations,” but that some models may “overestimate . . . the response to increasing greenhouse gas and other anthropogenic forcing.”

IPCC acknowledges that “due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”3 One wonders why such modesty did not accompany IPCC’s confident assertions in the 2007 AR4 that the warming observed from about 1980-1998 portended serious climate effects of GHG for the remainder of the century. In any event, the new SPM buries in footnote 16 an amazing admission: “No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.”4 Wow again. If we do not know how increases in GHG concentrations affect the climate, then the science really is not “settled,” is it? Should public policies be based upon such uncertainty?

Lousy Modeling Results Are a Guarantee of Future Performance

Let us turn now to the past performance of the climate models. A systematic analysis of their predictive track record was published recently in Nature Climate Science. The central conclusion for 1993-2012: “The observed rate of warming [of 0.14 ± 0.06 °C (95% confidence interval)] per decade is less than half of the [models’] simulated rise… of 0.30 ± 0.02 °C per decade (using 95% confidence intervals on the model average).” For 1998-2012, “the observed trend of 0.05 ± 0.08 °C per decade is more than four times smaller than the average simulated trend of 0.21 ± 0.03 °C per decade.”

With respect to IPCC’s predictions published in the assessment report series over time in the context of the recent temperature record, consider the following figure. Note that the more recent actual temperature record — let us shunt aside the variousproblems with the surface temperature measurement stations — lies either at the bottom or below the 95 percent confidence interval (“range of forecast”) defined in the 2007 AR4.

Another way to view the divergence between the predictions made by the climate models and the actual temperature record is displayed in the following figure.

The average prediction for the increase in surface temperatures since 1980 across 32 climate models participating in Phase 5 of the Coupled Model Intercomparison Project (a project of the World Climate Research Programme, sponsored jointly by the International Council for Science, the World Meteorological Organization, and the Intergovernmental Oceanographic Commission) is at least 71 percent higher than the actual record, again putting aside the problems inherent in the surface temperature measurements.

How Likely?

That is the context that will not be reported. Instead, the major news reports faithfully have reproduced the SPM assertion that “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”5 (Italics in the original) And: “Equilibrium climate sensitivity” — the effect over a century of a doubling of atmospheric GHG concentrations — “is likely in the range of 1.5°C to 4.5°C.” (Italics in the original) “Likely” in the language of IPCC means a likelihood of 66-100 percent, a level of confidence very different from the “extremely likely” (95-100 percent) verbiage prominent in the press release accompanying the SPM. As an aside, the precise meaning of the percentage likelihoods is far from clear, as they resemble statistical confidence intervals but cannot in fact be confidence intervals because they do not reflect findings from a statistical analysis of actual data; they are instead the results of simulations produced by models that reproduce past and present temperature patterns poorly. For discussions of the confusion inherent in the definitions of the various “likelihood” levels of IPCC pronouncements, see this and this.

Note that the “extremely likely” assertion is about warming since 1950, while the “likely” qualifier applies to prospective warming over the course of this century. That is a distinction unlikely (!) to be noticed by most journalists, for whom a summary is vastly preferred to the actual analysis, and for whom a summary of summarized summaries would be even better. More fundamentally, the new “likely” range for equilibrium climate sensitivity is the aforementioned 1.5°C to 4.5°C; but the range projected in the 2007 AR4 was 2°C to 4.5°C. So the projected range of future warming has increased since 2007, a fact that suggests less certainty about the effects of increasing GHG concentrations; but IPCC’s confidence about the dominance of the human impact on temperatures since 1950 has increased from “very likely” (90-100 percent confidence) to “extremely likely” (95-100 percent confidence). There it is in black and white: less confidence about future effects of GHG concentrations rests upon more confidence about the past. This is utter confusion, a reality underscored by the fact that the 2007 AR4 offered, in addition to the aforementioned range, a best estimate of 3°C of warming by the end of the century. The AR5 offers no such best estimate in addition to the (wider) range; the closest such projection in the AR5 is a “likely” (66-100 percent confidence) range of 0.3°C-0.7°C for 2016-2035 relative to 1986-2005, made with “medium confidence.”6

Words, Words, Words

For an amusing look at the workings of the IPCC sausage factory, the International Institute for Sustainable Development offers its Earth Negotiations Bulletin, a recent report of which summarizes the wrangling over the precise wording of the AR5 SPM. A small sampling:

Concerning the evidence that the key findings of the report are based on, Saudi Arabia suggested adding “assumptions” or “scientific assumptions” to the list. The addition of “scientific assumptions” was supported by Brazil and opposed by Austria, Canada, Germany and Belgium. The latter underscored that assumptions are already implicitly included in the already-listed theory, models and expert judgement. The Group rejected the insertion.

*****

On the headline statement, which states that warming of the climate system is unequivocal and, since 1950, many of the observed changes are unprecedented over decades to millennia, Saudi Arabia said the statement was “alarmist,” urged qualifying the terms “unequivocal” and “unprecedented,” requested using the year 1850 instead of 1950, and called for a reference to slowed warming over the past 15 years.

Germany, Australia, Chile, Spain, Fiji, New Zealand, the US, Saint Lucia, Tanzania, Mexico, Slovenia, the UK and others supported the statement as presented, with Germany pointing out that AR4 concluded almost the same. Canada pointed out that factors other than warming will be the emphasis in the future. The Russian Federation proposed “changing,” rather than warming of the climate system. After some discussion, Saudi Arabia agreed to accept the statement as presented.

*****

On precipitation, the discussion mainly centered around the question whether the text should focus on the Northern Hemisphere only. Guinea, Tanzania, Madagascar, Malawi, Peru, Trinidad and Tobago, and the Philippines highlighted the need to mention the Southern Hemisphere as well, since precipitation is an important issue for policy makers there. Mali underlined the importance of rain-based agriculture; Ethiopia highlighted droughts and floods that have occurred due to precipitation variability; and the Comoros stressed the special vulnerability of island states. A contact group developed a compromise to include two additional maps that show the changes in precipitation in 1901-2010 and 1951-2010.

And so on and so forth. This is “science.”

Benjamin Zycher is a visiting scholar at the American Enterprise Institute.